Wide band stop band filter including a ferrite region biased by a graded magnetic field

ABSTRACT

A stripline conductor is sandwiched between two ferrite slabs which, in turn, are sandwiched between two magnetic pole pieces. The pole pieces are in contact with opposite poles of a permanent magnet, placing the ferrite slabs in a magnetic field. The distance between the pole pieces varies along the length of the stripline, causing the magnetic field to be nonuniform. The ferrite slabs will absorb electromagnetic radiation at a frequency determined by the magnetic field strength and thus signals on the stripline will be absorbed in a frequency band determined by the range of magnetic field strengths along the length of the stripline.

[ 1 June 20, 1972 WIDE BAND STOP BAND FILTER INCLUDING A FERRITE REGION BIASED BY A GRADED MAGNETIC FIELD Inventors: Pedro A. Szente, San Jose; Robert Joly,

Palo Alto, both of Calif.-

Hewlett-Packard Company, Palo Alto, Calif.

Dec. 7, 1970 Assignee:

Filed:

Appl. No.:

US. Cl. ..333/73 R, 333/73 S, 333/84 M Int. Cl. ..H01p 3/08, 1-103h 7/04, H0311 13/00 Field ofSearch ..333/73, 73 S, 84, 84 M, 73 W,

References Cited UNITED STATES PATENTS 8/1960 Reeves ..333/73 3,095,546 6/1963 Ayres et a] ..333/24.2 3,534,299 10/1970 Eberhardt ..333/84 M X 3,521,196 7/1970 Alfandari et al.. ..333/73 R X 3,090,930 5/1963 Dunn ..333/24.l

Primary Examiner-Herman Karl Saalbach Assistant Eraminer-Marvin Nussbaum Attorney-Roland l. Griffin [57] ABSTRACT A stripline conductor is sandwiched between two ferrite slabs which, in turn, are sandwiched between two magnetic pole pieces. The pole pieces are in contact with opposite poles of a permanent magnet, placing the ferrite slabs in a magnetic field. The distance between the pole pieces varies along the length of the stripline, causing the magnetic field to be nonuniform. The ferrite slabs will absorb electromagnetic radiation at a frequency determined by the magnetic field strength and thus signals on the stripline will be absorbed in a frequency band determined by the range of magnetic field strengths along the length of the stripline.

7 Claims, 4 Drawing Figures PATENTEDmzo m2 3.671.888

sum 10F 2 :hll l figure 1 I ATTENUATION I l I 1 Odb I l I I I I I I 70 T1 T3 f f2 FREQUENCY l 2 INVENTORS PEDRO A. SZE

ROBERT JO PATENTEDJUHZO :572 3,871,888

sum 2 or 2 INVENTORS PEDRO A. SZENTE ROBERT JOLLY WIDE BAND STOP BAND FILTER INCLUDING A FERRITE REGION BIASED BY A GRADED MAGNETIC FIELD BACKGROUND AND SUMMARY OF THE INVENTION The absorptive properties of a ferrite, such as yttrium-irongarnet (YIG) in a magnetic field are well known in the art. See, for example, P. Bernardi, IEEE Transactions on Microwave Theory & Techniques, Vol. MTT-l7, No. 2, February 1969, page 62 et seq. YIG resonators have been used for sometime in microwave applications requiring narrow band resonators, and these resonators have been made tunable by using electromagnets to vary the magnetic field the YIG is in. Notch filters made with ferrite materials have the desirable property of maintaining a low standing wave ratio in the passband and stopband, since energy in the stopband is absorbed rather than reflected. However, with a DC or electrically tunable uniform magnetic field, such notch filters are typically limited to bandwidths of to 20 percent of the filter center frequency.

In accordance with the preferred embodiments of the I present invention a structure is provided for spatially varying the magnetic field the ferrite resonator is in and for enclosing a transmission line to carry signals to be filtered. One conductor of a transmission line is sandwiched between two slabs of ferrite; and the pieces of ferrite are, in turn, sandwiched between two magnetic pole pieces. Each pole piece is in contact with a pole of a permanent magnet and the distance between the pole pieces varies along the length of the stripline, thus creating a spatially varying magnetic field. The ground plane or return conductor for the transmission line may be on one or both of the ferrite slabs on a side opposite the first-mentioned conductor. It is possible with this structure to obtain bandwidths that approach 200 percent of the filter center frequency. Such a filter can also be used as a low pass filter, if none of the signal frequencies to be attenuated are greater than the upper frequency limit of the filter stop band.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cut away perspective view of a band stop filter according to a preferred embodiment of the present invention.

FIG. 2 is a graph of the frequency response of the band stop filter of FIG. 1.

FIG. 3 shows another preferred embodiment of the present invention.

FIG. 4 shows an exploded view of a band stop filter structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I shows a band stop filter 11 comprising ferrite slabs 10 and 12, magnetic pole pieces I4 and 16, a permanent magnet 18 and a conductor 20. Note that in this figure, some of the parts have been exaggerated in size for clarity. Conductor 20 is supported on ferrite slab 12. A second or ground plane conductor 22 is supported on ferrite slab I0, and there may also be a ground plane conductor 23 supported on a side of ferrite slab I2 opposite conductor 20. Conductor 20 and ground plane conductors 22 and 23 comprise a strip transmission line. Ground plane conductors 22 and 23 are ofien connected together by a structure holding filter l l or connectors attached to the conductors. Ferrite slabs 10 and 12 are trapezoidal in cross section with ends 24 and 28 of ferrite slabs 10 and 12 thicker than ends 26 and 30. The varying thickness of the ferrite slabs causes a variation in the separation of pole pieces 14 and 16 along the length of conductor 20. Since pole pieces 14 and 16 are in contact with opposite poles 32 and 34 respectively of permanent magnet 18, the varying separation of the pole pieces results in a nonuniform magnetic field along the length of conductor 20. Conductor 20 is made wider at end 28 than at end 30 to preserve a constant characteristic impedance along the length of the transmission line. Ferrite slab 10 is slightly shorter than ferrite slab 12 to enable connection of external conductors to conductor 20.

FIG. 2 is a plot of attenuation versus frequency for a band stop filter as illustrated in FIG. 1. The filter presents essentially no attenuation to a signal below a lower cutoff frequency f or above an upper cutoff frequency f, The maximum attenuation of the filter is presented to signals in the stopband between frequencies f and fl. The transition regions between and f, and between 11 and f are the filter skirts. The ferrite slabs absorb signals having a frequency in the stopband f, to j} and the absorption frequency is proportional to the strength of the applied DC magnetic field. Since the magnetic field is determined by the proximity of the pole pieces 14 and 16, the upper cutoff frequency f, is determined by the closest spacing and the lower cutoff frequency f by the farthest spacing of pole pieces 14 and 16. The steepness of the lower filter skirt f to 1' that is, the closeness of f, and fl,, is determined by the abruptness of the magnetic field cutoff at ends 24 and 28, and any fringing of the field lines tends to reduce that steepness. Notches 36 and 38 in pole pieces 14 and 16 respectively tend to reduce fringing of the magnetic field lines, thus making the lower filter skirt f, to j}, steeper. It can easily be seen that the band stop filter can be used as a low pass filter if the frequencies of signals to be attenuated are not greater than frequency An alternate configuration for the pole pieces and ferrite slabs is shown in FIG. 3. Each of pole pieces 1 l4 and 116 has a truncated, V-shaped groove, 104 and 106 respectively, in a face which is in contact with a ferrite slab 110 or 112. Ferrite slabs 110 and 112 are irregular hexagons in cross section, mating with the grooves 104 and 106 in pole pieces 114 and 116 and with each other. As in the structure of FIG. 1, a conductor 120 is supported on ferrite slab 112. This configuration has the advantage that fringing magnetic fields at the low field strength region of the ferrite slabs are eliminated since flats 100 and 102, parallel to conductor 120, place a lower limit on the magnetic field strength.

FIG. 4 is an exploded view of a complete filter assembly. Those elements illustrated in FIG. 1 that also appear in FIG. 4 are denoted by primed designators, e.g., 10'. Ferrite slabs I0 and 12 are attached to pole pieces 14' and 16 respectively and conductor 20 is supported on ferrite slab 12. Pole pieces I4 and 16' are held by a fixture so that ferrite slabs 10' and 12' are in contact. Microwave connectors 52 and 54 with center conductors 56 and 58 respectively screw into holes, such as hole 60 infixture 50. Center conductors 56 and 58 protrude through holes, such as hole 62, and are connected to the ends of conductor 20'. Magnet 18' is supported on pole piece 14' with pole 32' in contact therewith. A magnetic coupler 64 is in contact with pole 34' of magnet 18 and also with a magnetic coupler 66. Magnetic coupler 66 supports pole piece 16, completing the magnetic circuit illustrated as a U- shaped magnet 18 in FIG. 1.

In practice, conductor 20 and ground plane conductors 22 and 23 may be gold or a similar metal vacuum deposited on ferrite slabs l0 and 12. Pole pieces 14 and 16 may be a ferrous metal which is also gold plated to permit soldering the ferrite slabs to the pole pieces. Fixture 50 may be gold plated brass or similar metal which pole pieces 14' and 16 are soldered to. Magnetic couplers 64 and 66 may be made of a ferrous metal, such as steel, and fixture 50 and magnetic couplers 64 and 66 may be screwed together. Such a band stop filter has been constructed using magnesium ferrite slabs 1 inch long, 0.100 inch wide, 5 mils thick at one end, and 25 mils thick at the other. For this structure, the magnetic field between the pole pieces varies from about 2.8 kilogauss to 14 kilogauss. The lower cutoff frequency f is 8 GI-Iz, and the upper cutoff frequency f is 40 GI-Iz, giving a bandwidth of 32 GB: or approximately percent of the geometric center frequency.

Other configurations of a band stop filter can also be built using a microstrip, a coaxial transmission line, or a wave guide (in lieu of a stripline) as the transmission line. Furthermore, there need not be two ferrite slabs; all that is required is that there be ferrite in the electromagnetic field of the wave propagating along the transmission line and that the spatially nonuniform, DC, magnetic field, from, for example, a permanent magnet or an electromagnet and pole pieces have a component normal to the magnetic component of the electromagnetic wave. If the DC magnetic field is provided by an electromagnet, then the stop band can be changed by changing the magnetic field strength.

' We claim:

1. A band stop filter comprising:

a transmission line having a first and a second conductor electrically insulated from one another for propagating electromagnetic waves;

a piece of ferrite substantially filling the space between the first and second conductors of the transmission line in a region having a magnetic field component of an electromagnetic wave propagating along the transmission line; and

magnetic means for producing a magnetic field varying along the direction of propagation of the electromagnetic wave on the'transmission line and having a component normal to said magnetic field component of the electromagnetic wave, in the region containing the piece of ferrite. 2. A band stop filter as in claim 1 wherein the first conductor is a flat stripline having a first ferrite slab in contact with 'one side and a second ferrite slab in contact with the other side thereof and the second conductor is supported in part by a surface of the first ferrite slab opposite the stripline and a surface of the second ferrite slab opposite the stripline.

3. A band stop filter as in claim 2 wherein the magnetic means comprises a magnet and first and second magnetic pole pieces, the first magnetic pole piece being adjacent the surface of the first ferrite slab opposite the stripline and connected to a first pole of the magnet, the second magnetic pole piece being adjacent the surface of the second ferrite slab opposite the stripline and connected to a second pole of the magnet, and the separation of the first and second magnetic pole pieces varying along the length of the stripline.

4. A band stop'filter as in claim 3 including magnetic circuit means wherein the magnetic means is symmetrical about the first conductor; the first magnetic pole piece being in contact with the conductor on the surface of the first ferrite slab opposite the stripline and; the second magnetic pole piece being in contact with the conductor on the surface of the second ferrite slab opposite the stripline and connected to the second pole of the magnet via the magnetic circuit means.

5. A band stop filter as in claim 4 wherein the separation of the first and second magnetic pole pieces varies monotonically with distance along the stripline.

6 A band stop filter as in claim 4 wherein the separation of the first and second magnetic pole pieces is substantially the same at each extreme of the stripline .and increases to a greater, maximum value at a point approximately half way between the extremes of the stripline.

7. A band stop filter as in claim 5 wherein each magnetic pole piece has a notch in an end having the greatest thickness and nearest a ferrite slab for reducing fringing magnetic fields at the end of greatest separation between-the pole pieces.

l I! l l 

1. A band stop filter comprising: a transmission line having a first and a second conductor electrically insulated from one another for propagating electromagnetic waves; a piece of ferrite substantially filling the space between the first and second conductors of the transmission line in a region having a magnetic field component of an electromagnetic wave propagating along the transmission line; and magnetic means for producing a magnetic field varying along the direction of propagation of the electromagnetic wave on the transmission line and having a component normal to said magnetic field component of the electromagnetic wave, in the region containing the piece of ferrite.
 2. A band stop filter as in claim 1 wherein the first conductor is a flat stripline having a first ferrite slab in contact with one side and a second ferrite slab in contact with the other side thereof and the second conductor is supported in part by a surface of the first ferrite slab opposite the stripline and a surface of the second ferrite slab opposite the stripline.
 3. A band stop filter as in claim 2 wherein the magnetic means comprises a magnet and first and second magnetic pole pieces, the first magnetic pole piece being adjacent the surface of the first ferrite slab opposite the stripline and connected to a first pole of the magnet, the second magnetic pole piece being adjacent the surface of the second ferrite slab opposite the stripline and connected to a second pole of the magnet, and the separation of the first and second magnetic pole pieces varying along the length of the striplIne.
 4. A band stop filter as in claim 3 including magnetic circuit means wherein the magnetic means is symmetrical about the first conductor; the first magnetic pole piece being in contact with the conductor on the surface of the first ferrite slab opposite the stripline and; the second magnetic pole piece being in contact with the conductor on the surface of the second ferrite slab opposite the stripline and connected to the second pole of the magnet via the magnetic circuit means.
 5. A band stop filter as in claim 4 wherein the separation of the first and second magnetic pole pieces varies monotonically with distance along the stripline.
 6. A band stop filter as in claim 4 wherein the separation of the first and second magnetic pole pieces is substantially the same at each extreme of the stripline and increases to a greater, maximum value at a point approximately half way between the extremes of the stripline.
 7. A band stop filter as in claim 5 wherein each magnetic pole piece has a notch in an end having the greatest thickness and nearest a ferrite slab for reducing fringing magnetic fields at the end of greatest separation between the pole pieces. 